Seven open browser windows, a chiming smart phone, and an ongoing Skype call: these are the hallmarks of the modern-day, multitasking employee. “Organizations are becoming more complex and information technology is becoming more prevalent,” says Nicola Persico, a professor of managerial economics and decision sciences at the Kellogg School of Management. As such, workers are often asked to do more in the same amount of time—a challenge that invariably leads to doing more at the same time.

But should it? There are, of course, costs to such a strategy. “The idea that working on many things at the same time can actually slow you down—it’s not a new idea,” says Persico. In recent years especially, the downsides of multitasking have been broadcast far and wide. “Don’t Multitask: Your Brain Will Thank You” warns an article in Time. “Break the Multitasking Habit” encourages USA Today. Most of the warnings have focused on our limited cognitive resources: We can only do so much at once before everything starts to suffer—and there are mental costs associated with pivoting between projects too.

But Persico—along with Decio Coviello of HEC Montréal and Andrea Ichino of the University of Bologna—recently built a mathematical model that offers another reason for the productivity losses associated with multitasking. Mental costs aside, switching back and forth between projects is fundamentally inefficient. “If there is value in finishing a project”—as opposed to merely making progress—“then the best thing you can do is to finish one before you start the next one,” says Persico.

The Inefficiencies Compound

Consider a scenario in which you have two projects, each of which requires a solid five days of work to complete. If you switch back and forth between the two tasks equally, finishing them on the same day, each project will take you an average of two five-day workweeks to complete. But if you dedicate all of your time to one project before switching to the other, each project will take you, on average, just a week and a half (a week for the first and two weeks for the second). Stacking projects back to back, in other words, allows you to finish one project much earlier than you otherwise would, and takes you no longer to complete the second. As the number of projects increases, so do the efficiencies gained by tackling them one by one.

The inefficiencies, according to Persico’s model, have the further effect of demotivating workers. “If I juggle more,” says Persico, “then it means that the rewards of my efforts are going to be more and more postponed.” After all, a paycheck, or completion bonus—or even just a pat on the back—does not mean as much when it is a month away as when it is just few days away.

Throw multiple supervisors into the equation, and a problematic situation gets even worse. “As a worker,” says Persico, “you don’t really choose what to work on in isolation. You do so under the pressure of other people—maybe your co-workers, maybe your superiors—and as this uncoordinated set of pressures takes place, then naturally you will be driven to multitask.” If each of your supervisors lobbies you to work on her task first, you will be pushed to work on more projects simultaneously than if left to your own devices. But the lobbying becomes something of a vicious cycle: as the number of projects increases, so do the completion times, which in turn increases the importance of lobbying efforts in the future.

A Note to Managers

In addition to offering employees yet another reason to avoid multitasking, Persico’s research suggests potential drawbacks to organizational structures in which a single worker or group reports to multiple supervisors. There are plenty of reasons why an organization might be structured this way, of course—individual divisions may not have the need for their own legal team, or HR department, for instance—but, says Persico, “I think you have to carefully consider the trade-offs.” Workers are left “between a rock and a hard place” should one manager not observe (or care) how much a worker is required to do for another manager.

Persico’s research is based on the premise that the value of a project is only realized on its completion. But he acknowledges there are occasions when this is not true. “If I’m a contractor and I only get paid when a house is fully remodeled, then my best option is not to work on several houses at the same time—I’m better off finishing up the first one quickly and getting paid for it.” He continues: “But if, rather, I am a plumber and I am being paid on an hourly basis, then it makes no difference to me whether I task juggle or not.” Still, there may nonetheless be costs associated with switching between gigs: Even an hourly plumber loses time and money trekking from one job to another. And for many knowledge workers, the only valuable project is a finished one—if only because a client or supervisor cannot evaluate it until it is done.

In his research, Persico has quantified what most people already understand about multitasking: that workflow slows as projects increase. But now we are able to grasp this intuitive phenomenon on a mathematical level. “If I can make a comparison that is probably too ambitious,” Persico says with a laugh, “everybody intuitively understands that gravity makes things fall down. But there is still a great advantage in knowing the exact mathematical law that governs . . . the force that attracts bodies to each other."

Jessica Love is a writer and editor at the Kellogg School of Management. This article originally appeared in Kellogg Insight.

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